The present invention relates to the field of inspecting and maintaining of steam generators used in nuclear-powered power plants, and more particularly to power plants employing relatively small steam generators.
In the present state of the art of inspection and maintenance of relatively large steam generators used in nuclear-powered power plants, a robot-based system is employed. A commonly known robot called CECIL was designed and fabricated by Foster-Miller, Inc. of Waltham, Mass. for this purpose. The CECIL robot is a self-contained unit for operation in a commercial nuclear steam generator. Commercial steam generators are relatively large, and the entire CECIL robot is able to enter the commercial steam generator for inspection and maintenance thereof. Commonly conducted inspection and maintenance operations include visual inspection for damaged areas, water jet lancing for cleaning sludge deposits, core boring for obtaining sludge deposits, and scrubbing of internal parts. Future maintenance operations may include sludge deposit removal by application of laser energy and sludge deposit removal by electrolytic enhancement.
However, commercial steam generators are relatively large compared to steam generators used in nuclear-powered naval power plants. With naval nuclear steam generators, the steam generators themselves are too small for entry of a CECIL robot. More specifically, naval nuclear steam generators generally have 4 or 5 inch secondary side handholes and a nominal 21/2 inch wide divider lane between arrays of steam generator tubes in the steam generator. These dimensions of a naval nuclear steam generator are entirely too small for employing the CECIL robot or other known robots that enter the nuclear steam generator for inspection and maintenance operations. It would be desirable, therefore, to provide a robot-based system that would be useful for inspection and maintenance of the relatively small nuclear steam generators found in naval nuclear steam generators.
Even with a commercial steam generator, problems are associated with the use of a robot that is self-contained and operates completely inside the steam generator. For example, in use, the robot rests on the lower blowdown pipe which is located just above the tubesheet. Consequently, there is a drop of approximately 9 inches, and the robot rests below the line of sight through the hand hole. Since the robot rests on the blowdown pipe, it is in an area of sludge deposit buildup and an area of steam generator tubes that have a coating of sludge deposits. In this respect, it is known that blowdown pipes are often in poor condition with regard to their structural soundness, so that there is a substantial risk of blowdown pipe failure when the weight of the self-contained robot rests on the blowdown pipe. It would be desirable, therefore, to provide a robot-based system for inspection and maintenance of nuclear steam generators that does not subject blowdown pipes to risk of failure due to carrying the weight of the robot system.
Another problem associated with the robot system that completely enters the nuclear steam generator is the problem of decontamination when inspection or maintenance operation is completed. In such a case, the entire relatively large and complex self-contained robot must be decontaminated after use. It would be desirable, however, if only a limited portion of a robot-based inspection and maintenance system for nuclear steam generators need to be decontaminated after their use.
Still another problem associated with the use of a robot system that completely enters a nuclear steam generator is the contact that takes place between the conventional robot and steam generator tubes inside the steam generator. More specifically, the conventional robot depends upon touching and pushing contact with steam generator tubes for indexing the robot from tube to tube. Furthermore, rigid positioning of the conventional robot inside the nuclear steam generator depends entirely upon how tight or hard a push the robot's positioning feet exert on the steam generator tubes. The problems are accentuated when vibrations due to high pressure water lancing are transmitted back through the lance to the internal robot body and positioning feet which are in contact with the steam generator tubes.
In addition, steam generator tubes in naval nuclear steam generators are smaller in diameter and have thinner walls than steam generator tubes of commercial nuclear steam generators. With thinner wall tubes, denting and distortion become a problem.
Thus, it would be desirable to provide a robot-based inspection and maintenance system for nuclear steam generators that does not place unwanted stresses and strains on the steam generator tubes inside the steam generator.
In some naval nuclear plants the handhole is offset with respect to the divider lane in the steam generator. Because of the offset, the port of the handhole and the divider lane are not in alignment. This lack of alignment creates difficulties for a relatively large robot which, for this reason, cannot be successfully used for inspection and/or maintenance operations. It would be desirable, therefore, to provide means for inspection and/or maintenance of the interior of a steam generator that can be successfully employed in a steam generator whose handhole is offset from the divider lane.